1. Field of the invention
The present invention relates to a motor with armature teeth and a field magnet. More specifically, the present invention relates to a motor in which a rotational magnetic field is generated by applying voltages of respective phases being different in phase from each other to divided armature windings wound on a plurality of teeth of an armature so that a rotor, that is, the armature or the field magnet is rotated by means of magnetic interaction between the rotational magnetic field thus generated and the magnetic field of the field magnet.
2. Description of the prior art
One example of a conventional brushless motor is shown in FIG. 1. In this conventional brushless motor, a stator or armature 1 includes twelve teeth 2.sub.1 -2.sub.12 being formed in a radial manner, and divided windings U.sub.1 -U.sub.4, V.sub.1 -V.sub.4 and W.sub.1 -W.sub.4 of U, V and W phases are wound on the respective teeth 2.sub.1 -2.sub.12 as armature windings 3. On the other hand, a rotor or field magnet 4 includes eight magnetic poles 5. Thus, in FIG. 1, a conventional brushless motor of three phases and eight poles is constructed. Then, when voltages of respective phases are applied to the divided armature windings U.sub.1 -U.sub.4, V.sub.1 -V.sub.4 and W.sub.1 -W.sub.4 of the armature 1, respectively, a rotational magnetic field is generated by the divided armature windings U.sub.1 -U.sub.4, V.sub.1 -V.sub.4 and W.sub.1 -W.sub.4, and the field magnet 4 is rotated by means of magnetic attracting force and magnetic repelling force being generated between the respective magnetic poles 5 of the field magnet 4 and such a rotational magnetic field.
Generally, it is well known that the number T of the teeth 2 of the armature 1 is given by the following equation (1) in association with the number 2P of the magnetic poles 5 of the field magnet and the number Q of the phases of the divided armature windings 3. EQU T=PQ (1)
In the conventional brushless motor as shown in FIG. 1, since P=4 and Q=3, the number T of the teeth becomes T=12.
In addition, as well known, the number of coggings for each one rotation of the field magnet is equal to the least common multiple (LCM) of T and 2P, and the number of simultaneous coggings is equal to the greatest common measure (GCM) thereof. Therefore, in the example as shown in FIG. 1, twenty-four times of coggings occur for each one rotation of the field magnet 4, and four-times coggings occur simultaneously. Therefore, in the conventional brushless motor as shown in FIG. 1, combined cogging force becomes four-times of the cogging force for each magnetic pole of the field magnet 4, as shown in FIG. 2.
If such large cogging force occurs, smooth rotation of the field magnet is prevented and vibration takes place. Therefore, the combined cogging force is to be made small as possible. As a means for making the combined cogging force small, there is a method where magnetic force of each magnetic pole of the field magnet is weakened, or distance between respective magnetic poles of the field magnet and the teeth of the armature is made larger. However, in any approaches, efficiency and output power of a motor is reduced.
Therefore, the inventor of the present invention has proposed a novel motor in which the combined cogging force becomes small in U.S. Pat. No. 4,700,098 (Oct. 13, 1987) which was assigned to one of assignees of the present invention, Sanyo Electric Co., Ltd.. In the motor disclosed in U.S. Pat. No. 4,700,098, the teeth of armature are arranged at unequal pitch so as to avoid coincidence in timing of coggings respectively generated by the respective magnetic poles of the field magnet, whereby the number of simultaneous coggings becomes "1" and the combined cogging force is made smaller than the conventional motor as shown in FIG. 1.
However, in the motor disclosed in U.S. Pat. No. 4,700,098, since the teeth are arranged at unequal pitch, width of respective slots between the adjacent teeth becomes uneven, therefore, there was disadvantage that it is difficult to wind the divided armature windings. Such disadvantage is conspicuous in the case where the divided armature windings are wound by an automatic windier.